Debt Disclosure [Abstract] Schedule of Short-Term and Long-Term Debt [Table Text Block] Schedule of Debt [Table Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Inventories [Table Text Block] Schedule of Inventory, Current [Table Text Block] Prepayments and Other Current Assets [Table Text Block] Schedule of Other Current Assets [Table Text Block] Other Non-Current Assets [Table Text Block] Schedule of Other Assets, Noncurrent [Table Text Block] Other Current Liabilities [Table Text Block] Other Current Liabilities [Table Text Block] Other Non-Current Liabilities [Table Text Block] Other Noncurrent Liabilities [Table Text Block] Supplemental Statement of Cash Flows Information [Table Text Block] Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Commitments and Contingencies Disclosure [Abstract] Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Axis] Guarantee Entity and Type [Domain] Guarantee Entity and Type [Domain] [Domain] for Guarantee Entity and Type [Axis] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] ALLETE Clean Energy [Member] U.S. Water Services [Member] U.S. Water Services [Member] U.S. Water Services [Member] BNI Energy Reclamation Liability [Member] BNI Coal Reclamation Liability [Member] Reclamation liability for closing costs associated with BNI Coal mine and mine facilities. ALLETE Properties Development and Maintenance Obligations [Member] ALLETE Properties Performance and Maintenance Obligations [Member] Performance and maintenance obligations to governmental entities to construct improvements in various projects. Town Center District Capital Improvement Bonds [Member] Town Center Community Development District [Member] Community development district obligations for Town Center District. Palm Coast Park District Special Assessment Bonds [Member] Palm Coast Park Community Development District [Member] Community development district obligations for Palm Coast Park District. Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Letters of Credit [Member] Financial Standby Letter of Credit [Member] Surety Bonds [Member] Surety Bond [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Surety Bonds and Letters of Credit [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Collateral Guarantor Obligations, Collateral Held Directly or by Third Parties, Amount Amount of any assets held either as collateral or by third parties that, upon the occurence of any triggering event or condition under the guarantee, can be obtained and liquidated to recover all or a portion of the amounts paid under the guarantee. Estimated Obligation Guarantor Obligations, Maximum Exposure, Undiscounted Ownership Percentage of Benefited Property Community Development District Obligation, Ownership of Property Benefited Ownership percentage of property benefitied by a real estate development project funded by municipal capital improvement revenue and special assessment bonds. Annual Assessment Community Development District Obligation, Annual Assessment Amount of annual assessments based on ownership percentage of benefited property related to real estate development projects funded by municipal capital improvement revenue and special assessment bonds. Statement of Financial Position [Abstract] Accounts Receivable [Abstract] Accounts Receivable, Net, Current [Abstract] Accounts Receivable, Allowance Allowance for Doubtful Accounts Receivable, Current Common Stock [Abstract] Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract] Common Stock, Par Value Common Stock, No Par Value Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares Outstanding Common Stock, Shares, Outstanding Common Stock, Shares Issued Common Stock, Shares, Issued Income Tax Disclosure [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Components of Income Tax Expense (Benefit), Continuing Operations [Abstract] Current Tax Expense [Abstract] Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Total Current Tax Expense Current Income Tax Expense (Benefit) Deferred Tax Expense [Abstract] Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Investment Tax Credit Amortization Deferred Other Tax Expense (Benefit) Total Deferred Tax Expense Deferred Income Tax Expense (Benefit) Total Income Tax Expense Income Tax Expense (Benefit) Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Abstract] Effective Income Tax Rate Reconciliation, Amount [Abstract] Income Before Non-Controlling Interest and Income Taxes Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest Statutory Federal Income Tax Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent Income Taxes Computed at 35 percent Statutory Federal Rate Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Increase (Decrease) in Tax Due to: [Abstract] Increase (Decrease) in Tax [Abstract] Increase (Decrease) in Tax [Abstract] State Income Taxes – Net of Federal Income Tax Benefit Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount Production Tax Credits Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount Other Effective Income Tax Rate Reconciliation, Other Reconciling Items, Amount Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Uncertain Tax Positions [Abstract] Income Tax Uncertainties [Abstract] Gross Unrecognized Tax Benefits Unrecognized Tax Benefits Gross Unrecognized Tax Benefits That Would Favorably Impact Effective Income Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate New Accounting Pronouncements [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Improvements to Employee Share-Based Payment Accounting [Member] Accounting Standards Update 2016-09 [Member] Accounting Standards Update 2016-09 [Member] Consolidated Statement of Income [Axis] Income Statement Location [Axis] Consolidated Statement of income [Domain] Income Statement Location [Domain] Income Tax Expense (Benefit) [Member] Income Tax Expense [Member] Income Tax Expense [Member] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Maximum [Member] Maximum [Member] New Accounting Pronouncements [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Effect of the Adoption New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification Regulated Operations [Abstract] Regulatory Assets and Liabilities [Table Text Block] Schedule of Regulatory Assets and Liabilities [Text Block] Assets [Abstract] Current Assets [Abstract] Assets, Current [Abstract] Cash and Cash Equivalents Cash and Cash Equivalents, at Carrying Value Accounts Receivable (Less Allowance of $2.4 and $3.1) Accounts Receivable, Net, Current Inventories – Net Inventory, Net Prepayments and Other Other Assets, Current Total Current Assets Assets, Current Property, Plant and Equipment – Net Property, Plant and Equipment, Net Regulatory Assets Regulatory Assets, Noncurrent Investment in ATC Equity Method Investments Other Investments Long-term Investments Goodwill and Intangible Assets – Net Intangible Assets, Net (Including Goodwill) Other Non-Current Assets Other Assets, Noncurrent Total Assets Assets Liabilities and Shareholders' Equity [Abstract] Liabilities and Equity [Abstract] Liabilities [Abstract] Current Liabilities [Abstract] Liabilities, Current [Abstract] Accounts Payable Accounts Payable, Current Accrued Taxes Taxes Payable, Current Accrued Interest Interest Payable, Current Long-Term Debt Due Within One Year Long-term Debt, Current Maturities Other Other Liabilities, Current Total Current Liabilities Liabilities, Current Long-Term Debt Long-term Debt, Excluding Current Maturities Deferred Income Taxes Deferred Tax Liabilities, Net, Noncurrent Regulatory Liabilities Regulatory Liability, Noncurrent Defined Benefit Pension and Other Postretirement Benefit Plans Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent Other Non-Current Liabilities Other Liabilities, Noncurrent Total Liabilities Liabilities Commitments, Guarantees and Contingencies (Note 13) Commitments and Contingencies Shareholders’ Equity [Abstract] Stockholders' Equity Attributable to Parent [Abstract] Common Stock Without Par Value, 80.0 Shares Authorized, 51.0 and 49.6 Shares Issued and Outstanding Common Stock, Value, Issued Accumulated Other Comprehensive Loss Accumulated Other Comprehensive Income (Loss), Net of Tax Retained Earnings Retained Earnings (Accumulated Deficit) Total Shareholders’ Equity Stockholders' Equity Attributable to Parent Total Liabilities and Shareholders' Equity Liabilities and Equity Compensation and Retirement Disclosure [Abstract] Components of Net Periodic Benefit Expense (Income) [Table Text Block] Schedule of Net Benefit Costs [Table Text Block] Regulatory Matters [Text Block] Public Utilities Disclosure [Text Block] Equity Method Investments and Joint Ventures [Abstract] ALLETE's Investment in ATC [Table Text Block] Equity Method Investments [Table Text Block] Fair Value Disclosures [Abstract] Fair Value [Text Block] Fair Value Disclosures [Text Block] Reclassification of Prior Income Statement [Table] Schedule of Prior Period Reclassification Adjustments [Table] Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period. Operating Revenue to Operating Revenue – Utility [Member] Operating Revenue – Utility [Member] The amount of regulated operating revenues recognized during the period. Operating Revenue to Operating Revenue – Non-utility [Member] Operating Revenue – Non-utility [Member] The amount of unregulated operating revenues recognized during the period. Cost of Sales – Non-utility to Fuel, Purchased Power and Gas – Utility [Member] Fuel, Purchased Power and Gas – Utility [Member] Cost of electricity purchased as well as fuel used to generate electricity used in operations and sold during the period. Reclassification of Prior Income Statement [Line Items] Schedule of Prior Period Reclassification Adjustments [Line Items] [Line Items] for Schedule of prior period reclassification adjustments to conform the presentation of prior period financial statements with the presentation in the current period. Prior Period Reclassification Amount Prior Period Reclassification Adjustment Statement of Comprehensive Income [Abstract] Unrealized Gain on Securities, Income Tax Expense Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax Defined Benefit Pension and Other Postretirement Benefit Plans, Income Tax Expense Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax Goodwill and Intangible Assets Disclosure [Abstract] Goodwill and Intangible Assets [Text Block] Goodwill and Intangible Assets Disclosure [Text Block] Power Purchase Agreements [Table] Long-term Contracts for Purchase of Electric Power [Table] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event [Member] Subsequent Event [Member] Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Integrated Resource Plan [Member] Integrated Resource Plan [Member] Integrated Resource Plan which contains a utility's analysis of existing and future energy resource alternatives and a projection of customer cost impact by class [Member] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] MPUC [Member] MPUC [Member] Minnesota Public Utilities Commission [Member] Minimum [Member] Minimum [Member] Generation Type [Axis] Generation Type [Axis] Generation Type [Axis] Generation Type [Domain] Generation Type [Domain] [Domain] for Generation Type [Axis] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Wind Turbine Generators [Member] Natural Gas-Fired [Member] Natural Gas Processing Plant [Member] Counterparty Name [Axis] Counterparty Name [Axis] Counterparty Name [Domain] Counterparty Name [Domain] Square Butte [Member] Square Butte [Member] Square Butte [Member] Minnkota Power [Member] Minnkota Power [Member] Minnkota Power [Member] Tenaska [Member] Tenaska [Member] Tenaska Energy, Inc. and Tenaska Energy Holdings, LLC [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Jointly Owned by ALLETE and Dairyland Power Cooperative [Member] Power Purchase Agreements [Axis] Long-term Contract for Purchase of Electric Power [Axis] Power Purchase Agreements [Domain] Long-term Contract for Purchase of Electric Power [Domain] Square Butte PPA [Member] Long-term contract with Square Butte for the purchase of electric power. Tenaska PPA [Member] Tenaska PPA [Member] Tenaska PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Natural Gas PPA [Member] Generating Facility [Axis] Generating Facility Axis [Axis] Name of generating unit or facility [Axis] Generating Facility Unit Name [Domain] Generating Facility Unit Name [Domain] [Domain] for Name of generating unit or facility [Axis] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit [Member] Square Butte Coal-fired Unit located near Center, North Dakota Power Sales Agreements [Axis] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Axis] Power Sales Agreements [Domain] Long-term Contracts for Sale of Electric Power [Domain] [Domain] for Long-term Contracts for Sale of Electric Power [Axis] Minnkota Power PSA [Member] Minnkota Sales Agreement [Member] Minnkota Sales Agreement [Member] Power Purchase Agreements [Line Items] Long-term Contract for Purchase of Electric Power [Line Items] Generating Capacity (MW) Capacity of a generating facility, unit, or project in megawatts. Output Entitlement Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased PPA Counterparty Total Debt Outstanding Long-term Contract for Purchase of Electric Power, Amount of Long-term Debt or Lease Obligation Outstanding Cost of Power Purchased Utilities Operating Expense, Purchased Power under Long-term Contracts Pro Rata Share of PPA Counterparty Interest Expense Long-term Contract for Purchase of Electric Power, Interest Included in Contract Charges Contract Term (Years) Long-term Contract for Purchase of Electric Power, Term of Contract (Years) Term in years of long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Inventories – Net [Abstract] Inventory, Net [Abstract] Fuel Energy Related Inventory, Coal Materials and Supplies Inventory, Supplies, Net of Reserves Raw Materials Inventory, Raw Materials, Gross Work in Progress Inventory, Work in Process, Gross Finished Goods Inventory, Finished Goods, Gross Reserve for Obsolescence Inventory Valuation Reserves Total Inventories – Net Prepayments and Other Current Assets [Abstract] Prepaid Expense and Other Assets, Current [Abstract] Deferred Fuel Adjustment Clause Deferred Fuel Cost Restricted Cash Restricted Cash and Cash Equivalents, Current Other Other Assets, Miscellaneous, Current Total Prepayments and Other Current Assets Other Non-Current Assets [Abstract] Other Assets, Noncurrent [Abstract] Contract Payment Contract Payment Non-Current Carrying value of a non-current deferred asset related to a payment made as part of a long-term contract. Finance Receivable Financing Receivable, Gross Restricted Cash Restricted Cash and Cash Equivalents, Noncurrent Other Other Assets, Miscellaneous, Noncurrent Total Other Non-Current Assets Other Current Liabilites [Abstract] Other Liabilities, Current [Abstract] PSAs Power Purchase Agreement - Current Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a current liability. Other Other Accrued Liabilities, Current Total Other Current Liabilities Other Non-Current Liabilities [Abstract] Other Liabilities, Noncurrent [Abstract] Asset Retirement Obligation Asset Retirement Obligations, Noncurrent PSAs Power Purchase Agreement - Noncurrent Liability The fair value adjustment for the price of power per MW at market rates compared to those at contract rate at the time of acquisition through the end of the contract term present valued to acquisition date resulting in a noncurrent liability. Contingent Consideration Business Combination, Contingent Consideration, Liability, Noncurrent Other Other Accrued Liabilities, Noncurrent Total Other Non-Current Liabilities Loss Contingencies [Table] Loss Contingencies [Table] FERC [Member] FERC [Member] Federal Energy Regulatory Commission [Member] Return on Equity Complaint 1 [Member] FERC Complaint 1 [Member] FERC Complaint 1 [Member] Return on Equity Complaint 2 [Member] FERC Complaint 2 [Member] FERC Complaint 2 [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Requested Return on Equity Filed with the FERC by Third Party Requested Return on Common Equity Filed with Regulatory Organization The requested return on common equity, which would be dedicated to the utilities within the regulatory organizations jurisdiction. FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Common Equity FERC Authorized Return on Equity Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder FERC Authorized Return on Equity, Including Incentive Adder Proposed Return on Equity by Federal Administrative Law Judge Proposed Return on Common Equity by Regulatory Organization The proposed return on common equity by regulatory organization Proposed Return on Equity by Federal Administrative Law Judge Including Incentive Adder Proposed Return on Equity by Regulatory Organization, Including Incentive Adder Proposed Return on Equity by Regulatory Organization, Including Incentive Adder Business Combinations [Abstract] Acquisitions [Text Block] Business Combination Disclosure [Text Block] Recurring Fair Value Measures [Table Text Block] Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Financial Instruments [Table Text Block] Fair Value, by Balance Sheet Grouping [Table Text Block] Income Tax Expense [Table Text Block] Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Income Statement [Abstract] Operating Revenue [Abstract] Revenues [Abstract] Utility Regulated Operating Revenue Non-utility Unregulated Operating Revenue Total Operating Revenue Revenues Operating Expenses [Abstract] Costs and Expenses [Abstract] Fuel, Purchased Power and Gas – Utility Cost of Purchased Power Transmission Services – Utility Cost of Transmission Cost of Sales – Non-utility Cost of Goods and Services Sold Operating and Maintenance Operating Expenses Depreciation and Amortization Depreciation, Depletion and Amortization Taxes Other than Income Taxes Taxes, Other Total Operating Expenses Costs and Expenses Operating Income Operating Income (Loss) Other Income (Expense) [Abstract] Nonoperating Income (Expense) [Abstract] Interest Expense Interest Expense Equity Earnings in ATC Income (Loss) from Equity Method Investments Other Other Nonoperating Income (Expense) Total Other Expense Nonoperating Income (Expense) Income Before Non-Controlling Interest and Income Taxes Income Tax Expense Net Income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Less: Non-Controlling Interest in Subsidiaries Net Income (Loss) Attributable to Noncontrolling Interest Net Income Attributable to ALLETE Net Income (Loss) Attributable to Parent Average Shares of Common Stock and Per Share Data [Abstract] Earnings Per Share [Abstract] Basic (Shares) Weighted Average Number of Shares Outstanding, Basic Diluted (Shares) Weighted Average Number of Shares Outstanding, Diluted Basic Earnings Per Share of Common Stock Earnings Per Share, Basic Diluted Earnings Per Share of Common Stock Earnings Per Share, Diluted Dividends Per Share of Common Stock Common Stock, Dividends, Per Share, Declared Regulatory Matters [Table] Public Utilities General Disclosures [Table] Rate Type [Axis] Rate Type [Axis] Rate Type [Axis] including categories like electric rates, natural gas rates, water rates, etc. Rate Information by Type [Domain] Rate Information by Type [Domain] Rate Information by Type [Domain] Electric Rates [Member] Electric Rates [Member] Electric Rates [Member] Natural Gas Rates [Member] Natural Gas Rates [Member] Natural Gas Rates [Member] Water Rates [Member] Water Rates [Member] Water Rates [Member] PSCW [Member] PSCW [Member] Public Service Corporation of Wisconsin [Member] 2010 Minnesota Rate Case [Member] 2010 Rate Case [Member] Retail rate increase request filed with the Minnesota Public Utilities Commission (MPUC). Minnesota Cost Recovery Riders [Member] Current Cost Recovery Rider [Member] Current Cost Recovery Rider [Member] 2016 Minnesota General Rate Case [Member] 2016 Minnesota Rate Case [Member] [Member] 2016 Minnesota Rate Case [Member] [Member] FERC-Approved Wholesale Rates [Member] FERC-Approved Wholesale Rates [Member] Wholesale rates approved by the Federal Energy Regulatory Commission (FERC). Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider [Member] Renewable Cost Recovery Rider [Member] 2012 Wisconsin General Rate Case [Member] 2012 Wisconsin Rate Case [Member] 2012 retail rate increase requested filed with the Public Service Commission of Wisconsin (PSCW). 2016 Wisconsin General Rate Case [Member] 2016 Wisconsin Rate Case [Member] 2016 retail rate increase request filed with the PSCW [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Minnesota Power [Member] Minnesota Power [Member] Minnesota Power [Member] SWL&P [Member] SWL&P [Member] Superior, Water, Light and Power [Member] Customer Class [Axis] Customer Class [Axis] Customer Class [Axis] description of customer types or classes Customer Class [Domain] Customer Class [Domain] [Domain] for Customer Class [Axis] description of customer types or classes Retail Customers [Member] Retail Customers [Member] Retail Customers [Member] include residential customers, commercial customers and industrial customers. Municipal Customers [Member] Municipal Customers [Member] Municipal Customers [Member] include cities, governments and other municipalities Boswell Energy Center [Member] Boswell Energy Center [Member] Boswell Energy Center [Member] Long-term Contracts for Sale of Electric Power [Axis] Long-term Contracts for Sale of Electric Power [Domain] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Termination Effective June 2019) [Member] Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contract (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contracts (Cost-Based Formula Methodology for Entire Term) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Wholesale Electric Contracts (Expire December 2024) [Member] Business Segments [Axis] Segments [Axis] Business Segments [Domain] Segments [Domain] Corporate and Other [Member] Corporate and Other [Member] Regulatory Matters [Line Items] Public Utilities, General Disclosures [Line Items] Approved Return on Common Equity Public Utilities, Approved Return on Equity, Percentage Approved Equity Ratio Public Utilities, Approved Equity Capital Structure, Percentage Revenue from Cost Recovery Riders Regulated Operating Revenue, Other Requested Rate Increase (Decrease) Public Utilities, Requested Rate Increase (Decrease), Percentage Requested Return on Equity Public Utilities, Requested Return on Equity, Percentage Requested Equity Ratio Public Utilities, Requested Equity Capital Structure, Percentage Annual Additional Revenue Generated from Requested Final Rate Increase Public Utilities, Requested Rate Increase (Decrease), Amount Annual Additional Revenue Generated from Requested Interim Rate Increase Public Utilities, Interim Rate Increase (Decrease), Amount Annual Additional Revenue Generated from Requested Final Rate Increase, Amended Public Utilities, Requested Rate Increase (Decrease), Amended, Amount Expected Decrease in Annual Depreciation Expense Expected increase (Decrease) in Annual Depreciation Expense Based on a Regulatory Order Expected increase (decrease) in annual depreciation expense based on a regulatory order, if approved. Number of Customers Number of Customers Number of Customers Notice Required to Terminate (Years) Length of Notice Required to Terminate (Years) The length of notice required to terminate a contract (in years). Average Monthly Demand (MW) Average Customer Demand (MW) Average demand measured for a specified customer, measured in MW. Change in Annual Capacity Charge Per Contract, Percentage Change in Annual Capacity Charge Per Contract, Percentage The annual percentage change of the capacity charge specified in a power purchase, power sales, or electric sales agreement. Reduction in Operating Revenue due to Regulatory Outcome After-Tax Charge for North Dakota Investment Tax Credits Reversed Tax Adjustments, Settlements, and Unusual Provisions Acquisition of Non-Controlling Interest [Table] Noncontrolling Interest [Table] Acquisition of Non-Controlling Interest [Line Items] Noncontrolling Interest [Line Items] Payment to Acquire Non-Controlling Interest Payments to Acquire Additional Interest in Subsidiaries Gain or Loss in Net Income or Other Comprehensive Income Other Nonoperating Gains (Losses) Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] ALLETE Senior Unsecured Notes 3.11% Due 2027 [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Short-Term Debt - Principal Debt, Current, Gross Amount of short-term debt and current maturities of long-term debt before deduction of unamortized discount or premium and unamortized debt issuance costs. Short-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Debt, Current The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of short-term debt or the current maturities of long-term debt. Short-Term Debt - Total Debt, Current Long-term Debt - Principal Long-term Debt, Gross Long-Term Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense, Long-Term Debt Excluding Current Maturities The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of long-term debt excluding current maturities. Long-Term Debt - Total Total Debt - Principal Debt, Long-Term and Short-Term, Combined Amount, Gross Represents the aggregate of total long-term debt, including current maturities and short-term debt, before deduction of unamortized discount or premium and unamortized debt issuance expense. Total Debt - Unamortized Debt Issuance Costs Unamortized Debt Issuance Expense Total Debt - Total Debt, Long-term and Short-term, Combined Amount Proceeds from Issuance of Senior Unsecured Notes Proceeds from Issuance of Unsecured Debt Interest Rate Debt Instrument, Interest Rate, Stated Percentage Statement of Cash Flows [Abstract] Operating Activities [Abstract] Net Cash Provided by (Used in) Operating Activities [Abstract] Net Income AFUDC – Equity Increase (Decrease) in Allowance for Equity Funds Used During Construction Income from Equity Investments – Net of Dividends Income (Loss) from Equity Method Investments, Net of Dividends or Distributions Change in Fair Value of Contingent Consideration Business Combination, Contingent Consideration Arrangements, Change in Amount of Contingent Consideration, Liability Depreciation Expense Depreciation Amortization of PSAs Amortization of Power Purchase Agreements The amount of income recognized in the current period that reflects income associated with the reduced liability of the PPAs acquired in a business acquisition. As noncash income, this element is subtracted from net income when calculating cash provided by or used in operations using the indirect method. Amortization of Other Intangible Assets and Other Assets Other Depreciation and Amortization Deferred Income Tax Expense Share-Based and ESOP Compensation Expense Share-based Compensation Defined Benefit Pension and Postretirement Benefit Expense Pension and Other Postretirement Benefit Expense Bad Debt Expense Provision for Doubtful Accounts Changes in Operating Assets and Liabilities [Abstract] Increase (Decrease) in Operating Capital [Abstract] Accounts Receivable Increase (Decrease) in Accounts and Other Receivables Inventories Increase (Decrease) in Inventories Prepayments and Other Increase (Decrease) in Prepaid Expense and Other Assets Accounts Payable Increase (Decrease) in Accounts Payable Other Current Liabilities Increase (Decrease) in Other Current Liabilities Cash Contributions to Defined Benefit Pension Plans Pension Contributions Changes in Regulatory and Other Non-Current Assets Increase (Decrease) in Other Noncurrent Assets Changes in Regulatory and Other Non-Current Liabilities Increase (Decrease) in Other Noncurrent Liabilities Cash from Operating Activities Net Cash Provided by (Used in) Operating Activities Investing Activities [Abstract] Net Cash Provided by (Used in) Investing Activities [Abstract] Proceeds from Sale of Available-for-sale Securities Proceeds from Sale of Available-for-sale Securities Payments for Purchase of Available-for-sale Securities Payments to Acquire Available-for-sale Securities Investment in ATC Payments to Acquire Equity Method Investments Changes to Other Investments Payments for (Proceeds from) Other Investing Activities Additions to Property, Plant and Equipment Payments to Acquire Property, Plant, and Equipment Proceeds from Sale of Property, Plant and Equipment Proceeds from Sale of Productive Assets Cash for Investing Activities Net Cash Provided by (Used in) Investing Activities Financing Activities [Abstract] Net Cash Provided by (Used in) Financing Activities [Abstract] Proceeds from Issuance of Common Stock Proceeds from Issuance of Long-Term Debt Proceeds from Issuance of Long-term Debt Changes in Restricted Cash Proceeds from (Repayments of) Restricted Cash, Financing Activities Changes in Notes Payable Proceeds from (Repayments of) Notes Payable Repayments of Long-Term Debt Repayments of Long-term Debt Acquisition of Non-Controlling Interest Acquisition-Related Contingent Consideration Payments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements Dividends on Common Stock Payments of Ordinary Dividends, Common Stock Other Financing Activities Proceeds from (Payments for) Other Financing Activities Cash for Financing Activities Net Cash Provided by (Used in) Financing Activities Change in Cash and Cash Equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and Cash Equivalents at Beginning of Period Cash and Cash Equivalents at End of Period Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] Common Stock [Member] Common Stock [Member] Class of Stock [Line Items] Class of Stock [Line Items] Options to Purchase Shares Excluded from Computation of Diluted Earnings Per Share Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Employer Contributions to Defined Benefit Pension Plans - Common Stock Shares Stock Issued During Period, Shares, Employee Benefit Plan Employer Contributions to Defined Benefit Pension Plans - Common Stock Value Stock Issued During Period, Value, Employee Benefit Plan Reconciliation of Basic and Diluted Earnings Per Share [Abstract] Earnings Per Share Reconciliation [Abstract] Earnings Per Share - Basic [Abstract] Earnings Per Share, Basic [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Available to Common Stockholders, Basic Average Common Shares Earnings Per Share Earnings Per Share - Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Net Income Attributable to ALLETE Net Income (Loss) Available to Common Stockholders, Diluted Average Common Shares Earnings Per Share Dilutive Securities (Shares) Weighted Average Number Diluted Shares Outstanding Adjustment Document and Entity Information [Abstract] Document and Entity Information [Abstract] Document and Entity Information [Table] Document Information [Table] Document and Entity Information, Document [Axis] Document Information, Document [Axis] Document [Domain] Document [Domain] Document and Entity Information [Line Items] Document Information [Line Items] Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Entity Tax Identification Number Entity Tax Identification Number Current Fiscal Year End Date Current Fiscal Year End Date Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Current Reporting Status Entity Current Reporting Status Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Document Type Document Type Amendment Flag Amendment Flag Document Period End Date Document Period End Date Schedule of Business Acquisitions [Table] Schedule of Business Acquisitions, by Acquisition [Table] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Water & Energy Systems Technology [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] Water & Energy Systems Technology of Nevada, Inc. [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Schedule of Business Acquisitions [Table Text Block] Schedule of Business Acquisitions, by Acquisition [Table Text Block] Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract] Other Comprehensive Income (Loss) [Abstract] Other Comprehensive Income (Loss), Net of Tax [Abstract] Currency Translation Adjustments Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, Net of Tax Unrealized Gain on Securities Net of Income Tax Expense of $0.2, $0.3, $0.5 and $- Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.2, $0.1, $0.3, and $0.2 Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax Total Other Comprehensive Income Other Comprehensive Income (Loss), Net of Tax Total Comprehensive Income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Less: Non-Controlling Interest in Subsidiaries Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Total Comprehensive Income Attributable to ALLETE Comprehensive Income (Loss), Net of Tax, Attributable to Parent Reconciliation of Basic and Diluted Earnings Per Share [Table Text Block] Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Equity [Abstract] Reclassifications Out of Accumulated Other Comprehensive Income (Loss) [Text Block] Disclosure of Reclassification Amount [Text Block] Investments [Abstract] ALLETE Properties Real Estate Investments, Net Available-for-sale Securities Available-for-sale Securities, Noncurrent Cash Equivalents Other Restricted Assets, Noncurrent Other Other Long-term Investments Total Other Investments Available-for-sale Corporate and Governmental Debt Securities, Maturities [Abstract] Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract] One Year or Less Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value One Year to Less Than Three Years Available-for-sale Securities, Debt Maturities, After One Year Through Three Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the first fiscal year through the third fiscal year following the latest fiscal year (maturing in more than one year to less than three years). Three Years to Less Than Five Years Available-for-sale Securities, Debt Maturities, After Three Years Through Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing after the third fiscal year through the fifth fiscal year following the latest fiscal year (maturing in three years to less than five years). Five or More Years Available-for-sale Securities, Debt Maturities, After Five Years, Fair Value Amount of available-for-sale debt securities at fair value maturing in after the fifth fiscal year following the latest fiscal year (maturing in five years or more). Impairment of Land Inventory Impairment of Real Estate Revision of Prior Balance Sheet [Table] Schedule of Quantifying Prior Year Misstatement Corrected in Current Year Financial Statements [Table] Consolidated Balance Sheet [Axis] Balance Sheet Location [Axis] Consolidated Balance Sheet [Domain] Balance Sheet Location [Domain] Regulatory Assets [Member] Regulatory Assets [Member] Regulatory Assets [Member] Deferred Income Taxes [Member] Deferred Income Taxes [Member] Deferred Income Taxes [Member] Revision of Prior Balance Sheet [Line Items] Quantifying Misstatement in Current Year Financial Statements [Line Items] Immaterial Revision Quantifying Misstatement in Current Year Financial Statements, Amount Segment Reporting [Abstract] Business Segments [Table] Schedule of Segment Reporting Information, by Segment [Table] Regulated Operations [Member] Regulated Operations [Member] [Member] Operation of entities that are regulated by governmental organizations for example, but not limited to, local, state, county, country, or other regulatory organizations. Business Segments [Line Items] Segment Reporting Information [Line Items] Number of Reportable Segments Number of Reportable Segments Number of Operating Segments Number of Operating Segments Land in Minnesota (Acres) Area of Land Operating Revenue Net Income (Loss) Attributable to ALLETE Assets Earnings Per Share and Common Stock [Text Block] Earnings Per Share [Text Block] Leasing Agreements [Table] Schedule of Operating Leased Assets [Table] Leasing Agreements [Axis] Property, Plant and Equipment, Type [Axis] Leasing Agreements [Domain] Property, Plant and Equipment, Type [Domain] BNI Energy Dragline [Member] Exploration and Production Equipment [Member] Leasing Agreements [Line Items] Operating Leased Assets [Line Items] Minimum Lease Payments Due Annually Operating Leases, Future Minimum Payments Due, Next Twelve Months Termination Fee Termination Fee A lump sum payment in conjunction with a lease termination. Minimum Lease Payments Due for the Remainder of 2017 Operating Leases, Future Minimum Payments, Remainder of Fiscal Year Minimum Lease Payments Due in 2018 Operating Leases, Future Minimum Payments, Due in Two Years Minimum Lease Payments Due in 2019 Operating Leases, Future Minimum Payments, Due in Three Years Minimum Lease Payments Due in 2020 Operating Leases, Future Minimum Payments, Due in Four Years Minimum Lease Payments Due in 2021 Operating Leases, Future Minimum Payments, Due in Five Years Minimum Lease Payments Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Percentage of Voting Interests Acquired Business Acquisition, Percentage of Voting Interests Acquired Name of Acquired Entity Business Acquisition, Name of Acquired Entity Total Consideration Business Combination, Consideration Transferred Cash Payment to Acquire Business Payments to Acquire Businesses, Gross Working Capital Adjustments Goodwill, Purchase Accounting Adjustments Payment Due in April 2018 Business Combination, Consideration Transferred, Liabilities Incurred Assets Acquired [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] Cash and Cash Equivalents Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents Other Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Customer Relationships Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill Goodwill Goodwill Other Non-Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Total Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets Liabilities Assumed [Abstract] Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Non-Current Liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities Total Liabilities Assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities Net Identifiable Assets Acquired Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Tax Deductible Goodwill Business Acquisition, Goodwill, Expected Tax Deductible Amount Financial Covenants [Table] Required Ratio of Indebtedness to Total Capitalization Debt Instrument, Required Indebtedness to Total Capital Ratio Indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Actual Ratio of Indebtedness to Total Capitalization Debt Instrument, Actual Indebtedness to Total Capital Ratio Actual indebtedness to total capital ratio required to be maintained by most restrictive financial covenant. Inventories – Net [Policy Text Block] Inventory, Policy [Policy Text Block] Subsequent Events [Policy Text Block] Subsequent Events, Policy [Policy Text Block] New Accounting Standards [Policy Text Block] New Accounting Pronouncements, Policy [Policy Text Block] Land Inventory [Policy Text Block] Real Estate Held for Development and Sale, Policy [Policy Text Block] Available-for-sale Investments [Policy Text Block] Marketable Securities, Available-for-sale Securities, Policy [Policy Text Block] Acquisitions [Policy Text Block] Business Combinations Policy [Policy Text Block] Fair Value Measurement [Policy Text Block] Fair Value Measurement, Policy [Policy Text Block] North Dakota Investment Tax Credits [Policy Text Block] Regulatory Income Taxes, Policy [Policy Text Block] Regulatory Assets and Liabilities [Policy Text Block] Public Utilities, Policy [Policy Text Block] Equity Method Investments [Policy Text Block] Equity Method Investments, Policy [Policy Text Block] Income Tax [Policy Text Block] Income Tax, Policy [Policy Text Block] Uncertain Tax Positions [Policy Text Block] Income Tax Uncertainties, Policy [Policy Text Block] Earnings Per Share [Policy Text Block] Earnings Per Share, Policy [Policy Text Block] Power Purchase Agreements [Policy Text Block] Consolidation, Variable Interest Entity, Policy [Policy Text Block] Environmental Accruals [Policy Text Block] Environmental Costs, Policy [Policy Text Block] Business Segments [Policy Text Block] Segment Reporting, Policy [Policy Text Block] Supplemental Cash Flow Information [Abstract] Cash Paid During the Period for Interest – Net of Amounts Capitalized Interest Paid, Net Cash Paid During the Period for Income Taxes Income Taxes Paid, Net Noncash Investing and Financing Activities [Abstract] Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract] Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment Decrease in Accounts Payable for Capital Additions to Property, Plant and Equipment The decrease in accounts payable in the current period related to capital additions recognized in prior periods. Capitalized Asset Retirement Costs Increase (Decrease) in Asset Retirement Obligations AFUDC–Equity ALLETE Common Stock Contributed to the Pension Plans Unrecorded Unconditional Purchase Obligation [Table] Coal, Rail and Shipping Contracts [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Coal, Rail and Shipping Contracts [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Coal Supply and Transportation Agreements [Member] Coal and Transportation [Member] Coal and Transportation [Member] Coal, Rail and Shipping Contracts [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Minimum Annual Payment Obligation for Remainder of 2017 Unrecorded Unconditional Purchase Obligation, Due in Remainder of Fiscal Year Minimum Annual Payment Obligation in 2018 Unrecorded Unconditional Purchase Obligation, Due within Two Years Minimum Annual Payment Obligation in 2019 Unrecorded Unconditional Purchase Obligation, Due within Three Years Minimum Annual Payment Obligation in 2020 Unrecorded Unconditional Purchase Obligation, Due within Four Years Minimum Annual Payment Obligation in 2021 Unrecorded Unconditional Purchase Obligation, Due within Five Years Minimum Annual Payment Obligation Thereafter Unrecorded Unconditional Purchase Obligation, Due after Five Years Investment in ATC [Table] Schedule of Equity Method Investments [Table] Equity Method Investment [Axis] Investment, Name [Axis] Equity Method Investment [Domain] Investment, Name [Domain] ATC [Member] ATC [Member] ATC [Member] Sensitivity Analysis [Axis] Sensitivity Analysis [Axis] The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Domain] Sensitivity Analysis [Domain] [Domain] for The hypothetical effect on future earnings using different variables of assumptions. Sensitivity Analysis [Member] Sensitivity Analysis [Member] [Member] for The hypothetical effect on future earnings using different variables of assumptions. Expense Presentation [Axis] Expense Presentation [Axis] Expense Presentation [Axis] Expense Presentation [Domain] Expense Presentation [Domain] [Domain] for Expense Presentation [Axis] After-tax [Member] After-tax [Member] After-tax [Member] Investment in ATC [Line Items] Schedule of Equity Method Investments [Line Items] Ownership Percentage Equity Method Investment, Ownership Percentage Expected Additional Investment in 2017 Equity Method Investment, Expected Additional Investment Additional investments expected to be made during the current fiscal year but after the filing date, not already separately disclosed as part of current period/year-to-date investments or as a subsequent event. ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] ALLETE's Investment in ATC [Roll Forward] Equity Investment Balance as of December 31, 2016 Cash Investments Equity in ATC Earnings Distributed ATC Earnings Proceeds from Equity Method Investment, Dividends or Distributions Equity Investment Balance as of June 30, 2017 Authorized Return on Equity Equity Method Investment, Approved Return on Common Equity The approved percentage return on common equity dedicated to the utility plant in the investee accounted for under the equity method of accounting. Authorized Return on Equity, Including Incentive Adder Equity Method Investment, Approved Return on Common Equity, Including Incentive Adder The approved percentage return on common equity, including an incentive adder for participation in a regional transmission organization dedicated to the utility plant in the investee accounted for under the equity method of accounting. Proposed Return on Equity Equity Method Investment, Proposed Return on Common Equity The proposed return on common equity, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting. Proposed Return on Equity, Including Incentive Adder Equity Method Investment, Proposed Return on Common Equity, Including Incentive Adder The proposed return on common equity, including an incentive adder for participation in a regional transmission organization, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting. Basis Point Reduction on Approved Rate of Return on Common Equity Basis Point Reduction on Approved Rate of Return on Common Equity A sensitivity analysis using basis point reduction. Annual Effect on Future Equity Earnings in ATC Effect on Future Earnings, Amount Business Segments [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table Text Block] Fair Value of Financial Instruments [Table] Fair Value, by Balance Sheet Grouping [Table] Fair Value, Hierarchy [Axis] Fair Value, Hierarchy [Axis] Fair Value Hierarchy [Domain] Fair Value Hierarchy [Domain] Level 2 [Member] Fair Value, Inputs, Level 2 [Member] Fair Value of Financial Instruments [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-Term Debt, Including Long-Term Debt Due Within One Year - Carrying Amount Long-term Debt Long-Term Debt, Including Long-Term Debt Due Within One Year - Fair Value Long-term Debt, Fair Value Investments [Text Block] Investment [Text Block] Transmission [Axis] Transmission [Axis] Information about each transmission investment. Transmission [Domain] Transmission [Domain] Name of transmission investment. Great Northern Transmission Line [Member] Great Northern Transmission Line [Member] Great Northern Transmission Line between Manitoba and Minnesota's Iron Range [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Manitoba Hydro [Member] Transmission Line Length (Miles) Transmission Line Length (Miles) Length of a transmission line in miles. Transmission Line Capacity (kV) Transmission Line Capacity (kV) Capacity of a transmission line in kilovolts. Total Project Cost in the U.S. Estimated Capital Expenditures, Including Past Expenditures The estimated amount of certain capital expenditures related to environmental compliance, regulatory matters, or other commitments and/or contingencies. This estimate includes expenditures expected in future periods and expenditures made through the end of the current period. Total Project Costs Incurred to Date Capital Cost Spent to Date Capital cost spent to date for project. Project Costs Recovered from Counterparty Capital Costs Spent To Date, Recovered Capital Costs Spent To Date, Recovered Schedule of Intangible Assets [Table Text Block] Schedule of Intangible Assets and Goodwill [Table Text Block] Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities [Table] Schedule of Regulatory Assets and Liabilities [Table] Regulatory Asset [Axis] Total Regulatory Asset [Domain] Regulatory Asset [Domain] Deferred Fuel Adjustment Clause [Member] Deferred Fuel Costs [Member] Defined Benefit Pension and Other Postretirement Benefit Plans [Member] Pension and Other Postretirement Plans Costs [Member] Income Taxes [Member] Deferred Income Tax Charge [Member] Asset Retirement Obligations [Member] Asset Retirement Obligation Costs [Member] Cost Recovery Riders [Member] Conservation Improvement Program [Member] Conservation Improvement Program [Member] Conservation Improvement Program [Member] PPACA Income Tax Deferral [Member] Medicare Part D Regulatory Asset [Member] Discloses the amount of regulatory asset arising from the deferral of the retail portion of a tax charge taken in 2010 resulting from the Patient Protection and Affordable Care Act of 2010 (PPACA). Other Other Regulatory Assets (Liabilities) [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] Wholesale and Retail Contra AFUDC [Member] North Dakota Investment Tax Credits [Member] North Dakota Investment Tax Credits Subject to Refund [Member] North Dakota Investment Tax Credits Subject to Refund [Member] Plant Removal Obligations [Member] Removal Costs [Member] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities [Line Items] Regulatory Assets and Liabilities Currently Earning a Return Types of Net Regulatory Assets Earning Returns Current Regulatory Assets Regulatory Assets, Current Non-Current Regulatory Assets Total Regulatory Assets Regulatory Assets Non-Current Regulatory Liabilities Solar Energy Generation [Member] Solar Energy Generation [Member] Solar Energy Generation [Member] Solar Energy PPA [Member] Solar Energy PPA [Member] Solar Energy Provider [Member] Output Being Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased (MW) Long-Term Contract for Purchase of Electric Power, Share of Plant Output Purchased Business Segments [Text Block] Segment Reporting Disclosure [Text Block] Pension and Other Postretirement Benefit Plans [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Pension and Other Postretirement Benefit Plans [Axis] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Pension and Other Postretirement Benefit Plans [Domain] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Pension [Member] Pension Plan [Member] Other Postretirement [Member] Other Postretirement Benefit Plan [Member] Pension and Other Postretirement Benefit Plans [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Components of Net Periodic Benefit Cost (Income) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost [Abstract] Service Cost Defined Benefit Plan, Service Cost Interest Cost Defined Benefit Plan, Interest Cost Expected Return on Plan Assets Defined Benefit Plan, Expected Return on Plan Assets Amortization of Prior Service Credits Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Amortization of Net Loss Defined Benefit Plan, Amortization of Gains (Losses) Net Periodic Benefit Cost (Income) Defined Benefit Plan, Net Periodic Benefit Cost Employer Contributions to Defined Benefit Pension Plans - Cash Employer Contributions to Defined Benefit Plans Defined Benefit Plan, Contributions by Employer Expected Future Employer Contributions in 2017 Defined Benefit Plans, Estimated Future Employer Contributions in Current Fiscal Year Short-Term and Long-Term Debt [Text Block] Debt Disclosure [Text Block] Minnesota Solar Energy Standard [Member] Minnesota Solar Energy Standard [Member] Minnesota Solar Energy Standard [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Camp Ripley Project [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Purchased Output [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Minnesota Solar Energy Standard - Community Solar Garden Project - Owned and Operated [Member] Minnesota Solar Energy Standard - Overall Mandate Percentage Solar Energy Standard Mandate, Overall Mandate Percentage Minnesota's solar energy mandate for total retail electric sales, excluding sales to certain industrial customers, by the end of 2020. Minnesota Solar Energy Standard - Small Scale Solar Mandate Percentage Solar Energy Standard Mandate, Small Scale Solar Mandate Percentage Minnesota's solar energy mandate for small scale solar with photovoltaic devices with a nameplate capacity of 20 kilowatts or less by the end of 2020. Expressed as a percentage of the overall mandate. Minnesota Solar Energy Standard - Qualifying Capacity for Small Scale Solar Mandate (MW) Solar Energy Standard Mandate, Qualifying Capacity for Small Scale Solar Mandate The maximum amount of nameplate capacity of a solar device to qualify for counting towards the small scale solar mandate of Minnesota's solar energy standard. Minnesota Solar Energy Standard - Percentage of Overall Mandate Expected to be Met with Current Filings or Projects Solar Energy Standard Mandate, Percentage of Overall Mandate Expected to be Met with Current Filings or Projects The percentage of the solar energy standard overall mandate expected to be met with current filings and projects. Other Investments [Table Text Block] Investment [Table Text Block] CIP Annual Filing [Member] CIP Annual Filing [Member] Conservation Improvement Program Annual Filing [Member] CIP Spending Requirement CIP Spend Requirement Minnesota's Next Generation Act of 2007 introduced a spend goal of 1.5% of net gross operating revenues from service provided in the state on energy CIPs each year. Financial Incentive Reason for Business Acquisitions Business Combination, Reason for Business Combination Investment in ATC [Text Block] Equity Method Investments and Joint Ventures Disclosure [Text Block] Operations and Significant Accounting Policies [Text Block] Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Environmental Matters [Table] Environmental Matters [Table] Environmental Matters [Table] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Axis] Environmental Remediation Contingency [Domain] Environmental Remediation Contingency [Domain] NOV Consent Decree [Member] NOV Consent Decree [Member] NOV Consent Decree [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] Ozone NAAQS [Member] SO2 and NO2 NAAQS [Member] SO2 and NO2 NAAQS [Member] SO2 and NO2 NAAQS [Member] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Clean Water Act - Aquatic Organisms [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Coal Combustion Residuals [Member] Environmental Matters [Line Items] Environmental Matters [Line Items] [Line Items] for Environmental Matters [Table] Additional Wind Generating Capacity (MW) Estimated Costs of Compliance Site Contingency, Loss Exposure Not Accrued, Best Estimate Estimated Costs of Compliance Site contingency, Loss Exposure Accrued and Best Estimate of Loss Exposure in Excess of Accrual Amount of loss exposure accrued and the best estimate of reasonably possible loss exposure in excess of the accrual. Schedule of Finite-Lived Intangible Assets [Table] Schedule of Finite-Lived Intangible Assets [Table] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets by Major Class [Axis] Finite-Lived Intangible Assets, Major Class Name [Domain] Finite-Lived Intangible Assets, Major Class Name [Domain] Customer Relationships [Member] Customer Relationships [Member] Developed Technology and Other [Member] Other Intangible Assets [Member] Weighted Average [Member] Weighted Average [Member] Finite-Lived Intangible Assets [Line Items] Finite-Lived Intangible Assets [Line Items] Definite-Lived Intangible Assets [Roll Forward] Finite-lived Intangible Assets [Roll Forward] Beginning Balance Finite-Lived Intangible Assets, Net Amortization Amortization of Intangible Assets Ending Balance Remaining Useful Life (Years) Finite-Lived Intangible Asset, Useful Life Accumulated Amortization Finite-Lived Intangible Assets, Accumulated Amortization Estimated Annual Amortization Expense for Definite-Lived Intangible Assets [Abstract] Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract] Remainder of 2017 Finite-Lived Intangible Assets, Amortization Expense, Remainder of Fiscal Year 2018 Finite-Lived Intangible Assets, Amortization Expense, Year Two 2019 Finite-Lived Intangible Assets, Amortization Expense, Year Three 2020 Finite-Lived Intangible Assets, Amortization Expense, Year Four 2021 Finite-Lived Intangible Assets, Amortization Expense, Year Five Thereafter Finite-Lived Intangible Assets, Amortization Expense, after Year Five Schedule of Indefinite-Lived Intangible Assets [Table] Schedule of Indefinite-Lived Intangible Assets [Table] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets [Axis] Indefinite-lived Intangible Assets, Major Class Name [Domain] Indefinite-lived Intangible Assets, Major Class Name [Domain] Trademarks and Trade Names [Member] Trademarks and Trade Names [Member] Indefinite-lived Intangible Assets [Line Items] Indefinite-lived Intangible Assets [Line Items] Indefinite-Lived Intangible Assets [Roll Forward] Indefinite-lived Intangible Assets [Roll Forward] Beginning Balance Indefinite-Lived Intangible Assets (Excluding Goodwill) Ending Balance Intangible Assets [Abstract] Intangible Assets, Net (Excluding Goodwill) [Abstract] Total Intangible Assets Intangible Assets, Net (Excluding Goodwill) Total Intangible Assets, Amortization Income Tax Expense [Text Block] Income Tax Disclosure [Text Block] Statement of Stockholders' Equity [Abstract] Consolidated Statement of Shareholders' Equity [Table] Statement [Table] Equity Components [Axis] Equity Components [Axis] Total Shareholders' Equity [Domain] Equity Component [Domain] Retained Earnings [Member] Retained Earnings [Member] Accumulated Other Comprehensive Loss [Member] AOCI Attributable to Parent [Member] Consolidated Statement of Shareholders' Equity [Line Items] Statement [Line Items] Beginning Balance Consolidated Statement of Shareholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Comprehensive Income [Abstract] Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest [Abstract] Net Income Other Comprehensive Income – Net of Tax [Abstract] Currency Translation Adjustments Unrealized Gain on Securities Defined Benefit Pension and Other Postretirement Plans Total Comprehensive Income Common Stock Issued Stock Issued During Period, Value, New Issues Dividends Declared Dividends, Common Stock Ending Balance Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Member] Fair Value, Measurements, Nonrecurring [Member] Assets and Liabilities Measured at Fair Value on a Nonrecurring Basis [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Indicators of Impairment Asset Impairment Charges Recurring Fair Value Measures [Table] Recurring Fair Value Measures [Member] Fair Value, Measurements, Recurring [Member] Level 1 [Member] Fair Value, Inputs, Level 1 [Member] Level 3 [Member] Fair Value, Inputs, Level 3 [Member] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Axis] Contingent Consideration [Domain] Contingent Consideration [Domain] [Domain] for Contingent Consideration [Axis] Accretion [Member] Accretion [Member] Accretion expense [Member] Payments [Member] Payments [Member] Payments made that decrease contingent consideration liability [Member] Changes in Cash Flow Projections [Member] Changes in Cash Flow Projections [Member] Changes in Cash Flow Projections [Member] Recurring Fair Value Measures [Abstract] Assets [Abstract] Assets, Fair Value Disclosure [Abstract] Investments [Abstract] Available-for-sale – Equity Securities Available-for-sale Securities, Equity Securities, Noncurrent Available-for-sale – Corporate and Governmental Debt Securities Available-for-sale Securities, Debt Securities, Noncurrent Total Fair Value of Assets Assets, Fair Value Disclosure, Recurring Liabilities [Abstract] Liabilities, Fair Value Disclosure [Abstract] Deferred Compensation Deferred Compensation, Fair Value Disclosure This element represents the portion of the balance sheet assertion, Deferred Compensation, valued at fair value by the entity. U.S. Water Services Contingent Consideration Business Combination, Contingent Consideration, Liability Total Fair Value of Liabilities Liabilities, Fair Value Disclosure, Recurring Total Net Fair Value of Assets (Liabilities) Fair Value, Net Asset (Liability) Fair Value Hierarchy Transfers, All Levels Fair Value Transfers Between Levels, All Levels The amount of transfers in and out of levels 1, 2, and 3 of the fair value hierarchy during the period. Activity in Level 3 [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Beginning Balance Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value Activity in Level 3 Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Period Increase (Decrease) Ending Balance Transmission [Table] Transmission [Table] Presents detailed information about transmission investments. Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Manitoba Hydro PPA (expires 2035) [Member] Transmission [Line Items] Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Output Being Purchased (MW) Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased (MW) Share of plant output being purchased in megawatts under a long-term contract to purchase electricity from a production plant constructed and financed by a public utility district, cooperative, or other organization. Commitments, Guarantees and Contingencies [Text Block] Commitments Contingencies and Guarantees [Text Block] Pension and Other Postretirement Benefit Plans [Text Block] Pension and Other Postretirement Benefits Disclosure [Text Block]